Hydrocarbon mixtures containing these C3 hydrocarbons may be separated from the product resulting from the steam cracking of hydrocarbon feedstocks such as naphtha. The proportions of the components will depend upon the nature of the feedstock undergoing cracking, the cracking conditions, and any other upstream processing. Typically the C3 stream may contain 0 to 30% by weight propane, 0.1 to 10% by weight MAPD, possibly a small amount, generally less than 1% by weight, of C4 hydrocarbons, and the balance propylene. In some cases, some of the MAPD may be selectively hydrogenated to propylene in an upstream selective hydrogenation reactor. However, it is generally necessary to separate residual MAPD from propylene before the latter is used e.g. for the manufacture of polypropylene.
The separation of MAPD from propylene is conveniently effected by fractional distillation, typically at about 55-60.degree. C. and at a pressure of about 20 bar abs. The propylene is recovered as the overhead stream while the MAPD stream is recovered as the bottoms from the distillation column. Any propane in the feed is recovered with the MAPD as bottoms.
Since MAPD streams are unstable, representing an explosion risk, it is desirable to limit the concentration of MAPD in such a bottoms stream to no more than about 50% by weight: indeed it is often specified that the MAPD concentration should not exceed a somewhat lower level, e.g. 30% by weight.
In order to meet this latter requirement, it is often necessary, especially where the concentration of MAPD in the feed is relatively high and the propane content is less than that of the MAPD, to effect the fractional distillation such that the bottoms stream contains a large proportion of propylene. Since propylene is generally more valuable than propane, this is not economically desirable.
Thus where the MAPD is recovered from the bottoms stream, for example by solvent extraction, e.g. using a polar solvent such as dimethyl formamide, to provide a MAPD stream for subsequent processing, for example for the production of methyl methacrylate, the residual components of the bottoms stream, i.e. propane and/or propylene, could be recycled to the C3 stream being fed to the C3 separation stage producing the aforesaid bottoms stream. However such recycle is often not practical, especially where the recovery of the MAPD from the bottoms stream is effected at a location remote from the site of the C3 separation. As a consequence it is often necessary to use the residual components of the bottoms stream, after separation of the MAPD therefrom, as fuel. This is not an economic use of the more valuable propylene in that bottoms stream.
Where it is not desired to recover the MAPD from the bottoms stream, the bottoms stream can be recycled to the cracker as part of the feedstock thereto. However where the bottoms stream contains a large proportion of propylene, this represents a waste of the valuable propylene.